Tires are a complex structural combination of many components of various physical properties and of various compositions. Two of such components are a circumferential tire tread which is intended to be ground-contacting and pair of tire sidewalls which conventionally extend between the tire's spaced apart bead portions radially outward to the peripheral edges of the tire tread.
Tire treads and tire sidewalls are normally intended to present very different properties for a tire. For example, tire treads, which are intended to be ground contacting, are normally expected to provide physical properties such as, for example, traction and resistance to abrasion. For example, tire sidewalls, which are not intended to be ground contacting, are normally expected to provide physical properties such as, for example, resistance to flex fatigue and scuff resistance.
For this invention, it is desired to present a tire with a tread, and optionally its sidewalls, which contains silica reinforcement in which the silica reinforcement is provided as a pre-formed silica/coupler composite, namely a pre-hydrophobated precipitated silica.
In practice, various tires with silica reinforced rubber treads have been described and manufactured. Some tires have been mentioned which have silica containing rubber sidewall(s). Some tires have been mentioned which have both treads and associated sidewalls which are composed of silica containing rubber compositions.
Alkoxysilane based coupling agents (e.g. bis(3-trialkoxysilylalkyl)polysulfides) are often used to aid in coupling of precipitated silicas to diene-based elastomers in a rubber composition. However, such alkoxysilane moieties rely upon reaction with hydroxyl groups (e.g. silanol groups) contained on the surface of the precipitated silica which in turn produces an alcohol byproduct in situ within the rubber composition itself which therefore presents an alcohol evaporative emissions concern either during the manufacture of the tire or for the resultant tire itself.
In practice, tire tread and tire sidewall rubber compositions normally contain antidegradants which may be either amine based or phenolic based antidegradants. Amine based antidegradants are usually preferred because they are usually more effective in inhibiting atmospheric degradation of diene-based elastomers than phenolic based antidegradants. A widely used and generally effective amine based antidegradant is N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, which may sometimes be referred to as “6PPD”. However, the dimethylbutyl moiety of the 6PPD forms a methyl isobutyl ketone byproduct upon reacting in its intended role as antidegradant in the tire with oxygen and/or ozone which can present a methyl isobutyl ketone evaporative emissions concern for the resultant tire. In practice, a presence of an effective antidegradant in a tire sidewall rubber composition is normally considered to be necessary to retard atmospheric (e.g. ozone and oxygen) degradation of diene-based elastomers contained in the sidewall rubber composition. Normally, it is considered herein that a complete replacement of the 6PPD with either phenolic antidegradants or other amine based antidegradants is not considered as being as effective as using a significant amount of the 6PPD itself to prevent atmospheric degradation of diene-based elastomers in the sidewall rubber composition.
For this invention, it is desired to provide a tire with a combination of a tire tread and associated sidewalls of individual rubber compositions which has reduced, or substantially eliminated, alcohol and methyl isobutyl ketone evaporative emissions.
Accordingly, a tire with a tread and optionally a sidewall is provided which is comprised of individual rubber compositions which contain a pre-hydrophobated silica reinforcement with the sidewall, and optionally the tread, does not contain the 6PPD antidegradant.
For said pre-hydrophobation purposes, the precipitated silica may be pre-treated with a mercaptosilane and optionally an alkoxysilane prior to its addition to the tread and sidewall rubber compositions. One aspect of such pre-hydrophobation of the precipitated silica is to reduce, or substantially eliminate, evolution of alcohol during the actual mixing of the silica with the rubber compositions.
From a historical perspective, a tire may be prepared with a rubber component which contains a precipitated silica which is hydrophobated in situ within the elastomer host by addition of both a coupling agent, as an organomercaptosilane, and an alkyl silane. For example, see U.S. Pat. No. 4,474,908. However, for this invention, it is required that the precipitated silica is pre-hydrophobated to substantially reduce the hydroxyl group content on its surface prior to its addition to the rubber composition.
From a historical perspective, tires may also be prepared with a rubber component in which both a coupling agent, as an organosilyl polysulfide, and an alkylsilane are individually added to a silica-containing rubber composition to treat the amorphous silica in-situ within the rubber host with both a hydrophobating agent (the alkylsilane) and a silica coupler (the organosilyl polysulfide compound). For example, see U.S. Pat. No. 5,780,538.
From a historical perspective, tires may be prepared where one or more of its components (e.g. tread and sidewall) are silica reinforced wherein the silica is pre-treated with a coupling agent and, optionally, a substituted alkylsilane, prior to addition of the silica to the associated rubber composition with an expected result of reduction of alcohol evolution upon mixing the silica with the rubber composition. For example, see European patent publication EP-A-1 142 730 where the silica is pre-hydrophobated with an alkylsilane (Formula I therein) such as, for example, an alkoxyorganosilane which will release an alcohol byproduct upon reaction with alcohol groups (e.g. silanol groups) contained on the surface of a precipitated silica and an organomercaptosilane (Formula II therein) such as, for example, an alkoxyorganomercaptosilane which will also release an alcohol byproduct upon reaction with alcohol groups (e.g. silanol groups) contained on the surface of a precipitated silica.
Representative examples of such alkylsilanes, and particularly alkoxyalkylsilanes are provided in said European patent publication for the pre-hydrophobation of the silica (from its Formula I) are trichloro methyl silane, dichloro dimethyl silane, chloro trimethyl silane, trimethoxy methyl silane, dimethoxy dimethyl silane, methoxy trimethyl silane, trimethoxy propyl silane, trimethoxy octyl silane, trimethoxy hexadecyl silane, dimethoxy dipropyl silane, triethoxy methyl silane, triethoxy propyl silane, triethoxy octyl silane, and diethoxy dimethyl silane.
Representative examples of such organomercaptosilanes, and particularly alkoxyorganomercaptosilanes are provided in said European patent publication for the pre-hydrophobation of the silica (from its Formula (II) triethoxy mercaptopropyl silane, trimethoxy mercaptopropyl silane, methyl dimethoxy mercaptopropyl silane, methyl diethoxy mercaptopropyl silane, dimethyl methoxy mercaptopropyl silane, triethoxy mercaptoethyl silane, and tripropoxy mercaptopropyl silane.
In practice, it is recognized that precipitated silica aggregates are typically hydrophilic (water attracting) in nature and, in order to aid in dispersing the silica aggregates in various rubber compositions, it is sometimes desired to make the silica aggregates more hydrophobic (water repelling) in nature and therefore more compatible with the rubber. Accordingly, and as described in the aforesaid U.S. Pat. Nos. 4,474,908 and 5,780,538, a hydrophobating agent may be added to a rubber composition in addition to the precipitated silica to combine with the silica in-situ within the rubber host to make the silica more hydrophobic in nature.
However, it is considered herein, for tire tread and tire sidewall applications where properties are desired by silica reinforcement are often sought, and particularly where a good homogeneous dispersion in the rubber host is often sought, that an in-situ modification of the amorphous silica within a viscous rubber host on a hit and miss basis, under relatively harsh high sheer and high temperature conditions is a relatively inefficient procedure of modifying the amorphous silica for use in rubber compositions which are intended to be silica reinforced, particularly where both an organosilane and alkyl silane are used which would compete within the rubber composition for reaction sites on the silica surface and particularly where an alcohol byproduct is formed in situ within the rubber composition which is then available as an evaporative emission.
Accordingly in one aspect, it is proposed herein to provide a tire having a combination of both tread and sidewall components comprised of individual rubber compositions which contain particulate pre-hydrophobated precipitated silica aggregates where the silica aggregates are added to, or mixed with, the individual rubber compositions in a pre-hydrophobated form instead of more inefficiently subsequently hydrophobating the silica aggregates in situ within the elastomer host, thus essentially eliminating formation of an alcohol byproduct therefrom in situ within the rubber composition itself and thereby essentially eliminating such in situ formed alcohol as a subject of evaporative emission.
From a historical perspective, according to U.S. Pat. Nos. 5,708,069 and 5,789,514 a silica gel may be derived by hydrophobating a silica hydrogel with both an organomercaptosilane and alkyl silane and drying the product. The resulting hydrophobated silica gel may be blended with natural rubber and/or synthetic rubber. This invention is intended to be exclusive of recovered silica gels and is intended to be limited to precipitated silica aggregates.
Also, historically, according to U.S. Pat. No. 5,750,610, an organosilicate-modified silica gel may be hydrophobated with both an organomercaptosilane and alkyl silane and the dried treated organosilicate-modified silica gel blended with natural rubber and/or synthetic rubber. This invention is not intended to be directed to use of such modified silica gels which, for the purposes of this invention, are intended to be distinguished from rubber reinforcing precipitated silica.
A general description of silica gel and precipitated silica may be found, for example, in the Encyclopedia of Chemical Technology, Fourth Edition (1997), Volume 21, Kirk-Othmer in Pages 1020 through 1023.
The pre-hydrophobated, rubber reinforcing, precipitated silica aggregates for this invention are not intended to be said silica gels referenced in the above Encyclopedia of Chemical Technology. 
A further descriptive discussion of silica gels and precipitated silicas may be found, for example, in U.S. Pat. No. 5,094,829 which, as such reference, is intended to be incorporated here in its entirety.
A significant aspect of this invention is the preparation of a tire with reduced evaporative emissions which requires a combination of a structural aspect of a cooperative composite of combined tread and sidewall components of individual rubber compositions which contain pre-hydrophobated precipitated silica aggregates instead of precipitated silica aggregates which are hydrophobated in situ within the elastomer host taken in combination with requiring the sidewall rubber, and optionally also the tread rubber, to be exclusive of the 6PPD antidegradant and taken in combination with requiring the sidewall rubber itself to contain a significant content of low unsaturation EPDM rubber or brominated copolymer of isobutylene and p-methylstyrene to produce a tire with reduced evaporative emissions. It is considered that such combination of cooperative tire tread and sidewall components is a significant departure from past practice, particularly where evaporative emissions is taken into consideration for a tire with acceptable performance characteristics.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.